Field of the Invention
The present invention relates to an image forming apparatus including a developing device that develops an electrostatic latent image formed on an image bearing member of an electrophotographic copier, a laser beam printer, or the like, to develop a toner image.
Description of the Related Art
In conventional electrophotographic image forming apparatuses, when images with a low coverage rate are successively output, developer is stirred and rubbed for a while in a state where almost no toner is consumed and supplied in a developing device. For example, the developer is stirred by a developer conveyance screw that conveys the developer, and rubbed between a development sleeve and a doctor blade. As a result, an external additive provided to the toner for charge control and flowability control might be separated from the toner or embedded in a toner surface (hereinafter, also referred to as toner deterioration). The toner deterioration causes image quality degradation, such as a grainy effect, degrading printed image quality.
For example, Japanese Patent Application Laid-Open No. 2006-023327 and Japanese Patent Application Laid-Open No. 2000-310909 propose techniques that address this issue. More specifically, toner refreshing is performed by forcibly discharging deteriorated toner and supplying toner in an amount corresponding to the discharged amount, so that image quality is maintained.
When images with a low coverage rate are successively output, not only the image quality degradation due to the toner deterioration described above but also the following problem occurs. More specifically, when images with a high coverage rate are successively output immediately after the images with a low coverage rate are successively output, an image density largely fluctuates.
This is caused by a sharp change in a toner charging amount in the developing device due to switching from the successive low coverage rate image output to the successive high coverage rate image output. While the low coverage rate images are output, the toner charging amount is likely to be high due to excessive frictional charging between the toner and the carrier because the amount of toner exchanged in the developing device is small. On the other hand, while the high coverage rate images are output, the toner charging amount is likely to be low because a large amount of toner is consumed and supplied.
The refresh control discussed in Japanese Patent Application Laid-Open No. 2006-023327 and Japanese Patent Application Laid-Open No. 2000-310909 is effective against the image quality degradation due to the toner deterioration as a result of successively outputting the low coverage rate images. However, the refresh control might not be sufficiently effective against the density fluctuation caused by the change in the toner charging amount occurring when the low coverage rate image output is switched to the high coverage rate image output. This is because the two issues described above do not necessarily occur concurrently. More specifically, when the successive low coverage rate image output is performed, the conventional toner refresh control, executed at timing for preventing the image quality degradation due to the toner deterioration, might not be effective enough to prevent the image density fluctuation due to the change in the toner charging amount caused by switching of the coverage rates.
All things considered, an attempt to address the above two issues with the conventional refresh control only might lead to an unnecessarily toner consumption or an insufficient refreshing effect.